1. Field of the Invention
This invention relates to the efficient operation of engine aftertreatment systems, and more particularly relates to aftertreatment systems comprising a diesel particulate filter or a selective catalytic reduction system.
2. Description of the Related Art
Diesel emissions regulations are driving many modern diesel engine systems to utilize aftertreatment devices to clean up exhaust emissions downstream of the engine. These devices typically have the property that they cannot be reconfigured in real time, and therefore must be designed such that the engine system can meet emissions regulations at all operating points. In practical terms, this typically means that the aftertreatment devices are configured to treat the full engine exhaust at rated operation, or maximum load on the engine system.
While this method makes an emissions compliant engine, it produces an over-designed system that operates at a low efficiency in many operating conditions for many applications. Some examples are in selective catalytic reduction (SCR) systems, and diesel particulate filters (DPFs).
SCR systems are utilized to reduce NOx in the exhaust gas to nitrogen. The SCR system operates optimally when the engine out NOx comprises equal parts NO and NO2. The NOx coming out of a diesel engine is typically mostly NO, and a component configured active to NOx, specifically to convert NO to NO2, is often installed upstream of the SCR component. This upstream component may be a diesel oxidation catalyst (DOC). The DOC typically contains a platinum-based catalyst, and is usually designed to convert enough NO to NO2 that the SCR system can convert enough NOx at rated engine operation to meet emissions regulations. The result of this is that at many operating conditions, the DOC converts too much NO to NO2, resulting in excessive use of the SCR reagent (usually urea or ammonia) as the SCR system operates at non-optimal efficiency with the excess NO2.
Another inefficiency in SCR systems is that an SCR catalyst may require a certain temperature to convert sufficient NOx for the engine system to meet emissions constraints. However, in a cold start environment, there may be several components upstream of the SCR catalyst that must be heated up before the exhaust stream will reach the SCR catalyst at a temperature sufficient to heat the SCR catalyst up. While those emissions components may be important for meeting overall emissions, the engine system may be designed such that they only need to be utilized intermittently to achieve the emissions targets. In one example, a DPF may be upstream of the SCR catalyst. The DPF may be 95% efficient at trapping particulates, but the engine system may only need 80% trapping to meet the emissions targets.
Some DPF systems utilize a DOC to convert NO to NO2, and enhance oxidation of soot in the DPF during normal operation between oxygen-based regeneration events. In these systems, the DOC may be sized for a high flow rate of exhaust flow, and there may be excessive NO to NO2 conversion during lower flow rates. Excessive NO2 can exceed design limitations—for example a limitation on the amount of NO2 out of the tailpipe to control brown smoke. Further, as a DPF becomes loaded with soot, it may begin exerting excessive backpressure on the engine.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that provides for enhancing efficiency in an exhaust aftertreatment system. Beneficially, such an apparatus, system, and method would manage an exhaust stream to help an SCR system perform optimally, to assist a DPF in performing optimally, and/or minimize the time and fuel consumed in getting an SCR system up to operating temperatures.